The invention relates to the field of optical fiber communication systems and more particularly to a lumped Raman amplifier with gain control and a method of controlling the gain of a Roman amplifier.
In optical fiber communication systems, optical signals propagating along an optical fiber undergo signal attenuation due to absorption and scattering in optical fibers. Therefore, optical signals require periodic amplification over long distances, which can be performed either by electrical repeaters or by optical amplifiers. Known optical amplifier types include Erbium-doped fiber amplifiers (EDFAs), semiconductor optical amplifiers (SOAs) and Raman amplifiers. Due to their flat gain over a wide signal wavelength band, Raman amplifiers have gained increasing attention in the recent past as ideal amplifier candidate for use in wavelength division multiplex (WDM) transmission systems.
The Raman amplification process is based on the Raman effect, which is a non-linear optical process that only occurs at high optical intensities and involves coupling of light propagating through the non-linear medium to vibrational modes of the medium, and re-radiation of the light at a different wavelength. Re-radiated light upshifted in wavelength is commonly referred to as a Stokes line, whereas light downshifted in wavelength is referred to as an Anti-Stokes line. The Raman effect is described by quantum mechanics as scattering of photons at molecules which thereby undergo a transition of their vibrational state. Raman amplification involves stimulated Raman scattering, where the incident beam, having a higher optical frequency, often referred to as the pump beam, is used to amplify the lower frequency beam often referred to as the Stokes beam or the signal beam through the Raman effect.
It is particularly important for all types of optical amplifiers to have a flat gain curve over the entire spectral band of the data signal to be amplified. Moreover, the gain must remain stable over time, which may be particularly difficult when the number of wavelengths in the data signal (i.e., the “channel loading”) changes. Known EDFAs have therefore an optical filter which is adapted to flatten the gain spectrum of the amplifier. Some EDFAs have additionally a variable optical attenuator (VOA), which is tuned when the input power varies to keep the gain flatness. Moreover, the optical pump power of EDFAs is increased with the number of signal wavelengths (or channels). Such a gain control function is however not known for lumped Raman fiber amplifiers (LRFAs).
U.S. Pat. No. 6,384,962 describes a method and apparatus for automatic gain equalization using Raman amplifiers. The apparatus contains a first and a second Raman amplifier, having first and second gain curves, respectively. The amplifiers are chosen such that the gain curve of the first amplifier has maxima where the gain curve of the second amplifier has minima and vice versa to achieve total gain curve which is substantially flatter than the first or second alone.
U.S. Pat. No. 6,456,425 describes a method and apparatus to perform lumped Raman amplification. In order to increase the total bandwidth of a multi-pump Raman amplifier, the optical signal is split into a plurality of signal parts. Each part is amplified using a separate Raman amplifier. Each amplified part is then combined into a single amplified optical signal.
U.S. Pat. No. 6,519,082 discloses a method of dynamically monitoring the on-off gain of a Raman amplifier using a probe signals outside the spectral range of the data signal. However, such off-band measurement cannot predict the gain characteristic within the spectral band accurately.
It is an object of the present invention to provide a method and related apparatus for controlling the gain of a Raman amplifier to compensate for span loss variations and changing channel loading.